Free radical initiated low density polyethylene has, historically, been polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure free radical initiated, low density polyethylene is highly complex. The permutations in the arrangement of their simple building blocks are essentially infinite. Free radical initiated LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. Free radical initiated LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
With recent developments in transition metal catalyst chemistry, low density polyethylene can now be produced by copolymerizing ethylene with various alpha-olefins. These linear low density polyethylene (LLDPE) resins generally possess little, if any, long chain branching. They are short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
U.S. Pat. No. 4,302,566, in the names of F. J. Karol et al. and entitled Preparation of Ethylene Copolymers in Fluid Bed Reactor, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio in the range; of from greater than or equal to 22, to less than or equal to 32, and a relatively low residual catalyst content, can be produced in granular form at relatively high productivities, if the monomer(s) are copolymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. Pat. No. 4,302,565, in the names of G. L. Goeke et al. and entitled Impregnated Polymerization Catalyst, Process For Preparing, and Use for Ethylene Copolymerization discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow rate in the range of from greater than or equal to 22, to less than or equal to 32, and a relatively low residual catalyst content can be produced in granular form at relatively high productivities, if the monomer(s) are copolymerized in a gas phase process with a specific high-activity Mg-Ti containing complex catalyst which is impregnated in a porous inert carrier material.
According to another technique, ethylene homopolymers having a density in the range of from about greater than or equal to 0.958 to less than or equal to 0.972, and a melt flow ratio in the range of from about greater than or equal to 22, to about less than or equal to 32, which have a relatively low residual catalyst residue, can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process, if the ethylene is homopolymerized in the presence of a high-activity Mg-Ti containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced are useful for a variety of end use applications.
The polymers as produced, for example, by the process of said patents and technique using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, in the range of from about greater than or equal to 2.7, to less than or equal to 4.1.
Over the years, film extrusion equipment has been optimized for the rheology of free radical initiated LDPE. The different molecular architecture of LLDPE results in a film processing behavior which requires different extrusion parameters. Although LLDPE resins can be extruded on equipment designed for free radical initiated LDPE resins, certain equipment modifications are often required in order to extrude the LLDPE resins at optimum conditions and at rates comparable to the high pressure resins. This is particularly true during extrusion of LLDPE which is processed into film.
Processing of granular polyolefins in single screw extruders involves feeding, conveying, melting, compressing, pumping, and homogenizing the material as it passes from one end of the extruder to the other. Bubble formation in the extrudate can arise when a critical level of volatiles/gases become trapped during the melting stage of extrusion and are not allowed to vent to atmosphere back through the extruder hopper. These volatiles/gases can originate in the resin and/or concentrate/or in the additives which are blended with the resin. They can also be generated during the extrusion process, evolving for example from thermally induced breakdown of concentrate/additive/resin components. The problems incident to the production of bubble free product from "powdered" polyolefins are discussed in U.S. Pat. No. 4,248,819 issued Feb. 3, 1981. According to the disclosure of the patent, the polyolefin powder is heated to a temperature of between 40.degree. C. and its melting point, and is introduced into a gap between two gap-forming elements. In this gap, the powder is compressed with a pressure of from 0.1 to 10 t per cm of gap length, and the powder so compressed is then comminuted.
It is noted that according to the patentee, the problems are incident to the use of "powdery" materials as contrasted to the use of "granular" materials.
According to the present invention, however, the term granular resin generally refers to the same type resin as defined by the term "powdery" in the U.S. Pat. No. 4,248,819.
In film extrusion, and particularly, film extrusion of LLDPE resins, bubbles formed as described above become elongated during melt extension between the die and the point where the extrudate undergoes a phase transformation to the solid state (frost line). The distorted bubbles are "frozen" in the fabricated film, taking on the shape of lenses (in crossection). These lens defects are relatively clear elongated blisters within the film. Under a microscope, one normally observes an elongated bubble with two distinct surfaces although in some cases, one of the surfaces may be ruptured by gases escaping from the bubble. The number, the density, and the size distribution of lenses are, in general, entirely random across the body of the film. Lens dimensions may vary from a few hundredths inch to over one inch in length, depending on the severity of the prevailing lensing conditions. At high concentrations, lenses can weaken the film and affect end use performance. At low concentrations, lenses affect primarily aesthetics.
Extrudate bubble formation (leading to lenses in fabricated film) during the extrusion of granular polyolefins is a complex multivariate process. Numerous factors are involved in lens formation. Both fabrication parameters and material parameters have been found to be important. Factor interactions further complicate the phenomenon.
Granular polyolefins, and particularly, granular LLDPE, generally exhibit a different melting behavior in extrusion than polyolefin pellets. The melting mechanism of pellets is governed by a thin layer of melt between the solid bed and the barrel, where shear generates heat to melt the top of the solid bed. When processing pellets, volatiles/gases can escape over the trailing edge of the screw and vent out the extruder hopper. In the case of granular polyolefins however, melting is generally more rapid. Volatiles/gases are trapped when the granular solid bed becomes encapsulated by the surrounding melt pool.
Granular polyolefins can have substantial porosity and high surface area relative to resin pellets. Increased air entrainment during extrusion is therefore a possibility.
A rheological consequence of the narrow molecular weight distribution characterizing these linear low density ethylene hydrocarbon copolymers, is that in the shear rate range common to most single screw extruders (100-5000 sec.sup.-1), these materials exhibit limited shear thinning behavior. The practial melt viscosity obtained in the extruder in processing these materials, is high relative to that obtained with long chain branched, free radical initiated, high pressure, low density polyethylene (at equivalent melt index).
The high shear viscosity of narrow molecular weight distribution linear low density ethylene hydrocarbon copolymers results in their processing at higher temperatures than equivalent melt index, long chain branched, free radical initiated low density polyethylene. Common processing temperatures for linear low density ethylene hydrocarbon copolymer film resins in blown film extrusion are approximately 204.degree. C. or greater whereas free radical initiated, high pressure, low density polyethylene resins generally process at temperatures of approximately 177.degree. C. Higher processing temperatures can cause thermal decomposition reactions in color concentrates/additives components/resin, resulting in evolved volatiles/gases.
In general, films extruded from polyethylene can be modified with various additive systems. They are generally formulated for blocking resistance and surface handling characteristics by the addition of antiblock agents (e.g. diatomaceous earth compounds). These additives also can contribute volatiles/gases to the extrusion system.
Pigment thermal stability is a primary factor in determining the lensing resistance of color concentrates used in film extrusion. Pigments suitable for coloring free radical initiated, high pressure low density polyethylene may not endure the thermal environment experienced in the film extrusion of narrow molecular weight distribution linear low density ethylene hydrocarbon copolymers. Color concentrate loading can have a dramatic effect on lens formation. Increased levels of a volatiles source will generally aggravate the problem of bubble formation in an extrudate, and, thus, increase lensing in fabricated film.
Volatiles/gases can arise from other sources. Residual/hydrocarbons in the resin, oligomeric species, catalyst residues and byproducts, etc. can also contribute to bubble formation in a melt.
A general approach to reduce bubble formation in an extruded melt is to reduce volatiles/gases fed to the extrusion system and minimize volatiles/gases generated in the extrusion process. Volatiles reduction, as a practicality, is difficult and/or undesirable. The ingredients commonly fed to an extruder processing polyethylene film can be potential sources of undesired volatiles/gases. Furthermore, as extrusion equipment becomes larger in size, bubble formation leading to film lensing is more prevalent, (e.g. 6 in. diameter extrusion equipment). Fabrication concepts to reduce volatiles include vented extrusion, utilization of a vacuum hopper, screw designs which develop early back pressure, etc. Some of these fabrication approaches are discussed in a paper by Domininghaus, H. "Problems in Processing Polyolefins Powders", SPE Antec Proceedings, pp 523-529, 1971.
It has now been found, unexpectedly, that resin formulation adjustments can raise the threshold levels of volatiles in the extrusion system before lensing occurs. As stated previously, film extruded from polyethylene can be modified with various pigment systems. They are also, generally, formulated for blocking resistance and surface handling characteristics by the addition of antiblock agents. Common antiblock agents include particulate, inorganic matter such as ground diatomaceous earth compounds (SUPER FLOSS), precipitated silicas (syloids), calcium carbonate, talc, etc. The presence of this particulate, inorganic matter, has been found to aggravate lensing. The presence of particulate inorganic antiblock agents causes discrete bubbles to form as the formulated melt decompresses on exiting the film die. Elimination of particulate, inorganic antiblocking agents from granular LLDPE film formations has been found to significantly reduce the occurrence of extrudate bubble formation which, in turn, leads to reduced lens formation in fabricated film. It has been further found that a specific agent provides films which are significantly reduced in the incidence of lens formation while maintaining all the advantages of the antiblock agent characteristics.
Thus according to copending U.S. patent application Ser. No. 462,432 filed on Jan. 31, 1983 and assigned to a common assignee, there is provided a method for reducing lensing in films fabricated from a granular linear narrow molecular weight distribution ethylene hydrocarbon copolymer containing inorganic or mixtures of inorganic and organic antiblock agents and other additives under conditions which would otherwise produce lensing in said films, which comprises, forming a film forming composition which includes a granular linear low density ethylene hydrocarbon copolymer resin and from about 0.05% to about 0.5% by weight of N,N'-ethylene-bis-stearamide, said stearamide replacing all or part of inorganic antiblock agent, extruding said film composition through a film die, and thereafter discharging said composition from said die to form a film of said extruded composition.
The present invention provides an improvement in part over the method disclosed in the copending application Ser. No. 462,432 by incorporating a specific type silica alumina ceramic microsphere filler, in designated amounts to the composition disclosed therein.